Current InP-based HBTs typically employ InP or InGaAs as the collector. InGaAlAs and InGaAs/graded-InGaAsP/InP composite collectors have also been utilized. No reported literature, however, makes reference to further using the collector material for waveguide applications. InGaAs, which is a commonly used collector material in InP-based HBTs, absorbs light with wavelengths shorter than 1.5 microns, and hence cannot practically be used as a waveguide material. InP is transparent to 1.5 micron light, but cannot be easily clad with a semiconductor to confine light to thereby serve as a waveguide.
For a DHBT to function satisfactorily, there should be no spike in the conduction band between the base and the collector. As appreciable by those skilled in the art, the compositions of the alloys forming the base and the collector that enable transistor operation are not obvious and must be experimentally derived for each particular application. Controlling the composition of thick (˜2000 Å thickness used in the collector) mixed group-V alloys in order to maintain good lattice matching with the InP substrate is likewise not a straightforward matter.
What is needed is a method and apparatus for providing DHBTs with good waveguide properties to enable the manufacture of highly-integrated opto-electronic devices. The embodiments of the present disclosure answer these and other needs.